The present invention relates to a method for detecting gaseous materials present in the atmosphere of a selected area and to apparatus useful therefor.
In production facilities, particularly chemical production facilities, potentially hazardous materials are often employed. To reduce the risk of production, handling and use of such materials, detection of the presence of their vapors in the atmosphere before dangerous levels are reached is desirable. Continuous monitoring of the atmosphere of the production facilities is one way to ensure early detection of undesirable materials in the atmosphere.
Many approaches for monitoring the atmosphere for the presence of pollutants have been explored. U.S. Pat. No. 3,766,380 and U.S. Pat. No. 3,925,666 for example, disclose methods in which an infrared laser beam is transmitted through the atmosphere. The reflections of this transmitted laser beam are collected and analyzed for information relating to the presence of gaseous pollutants. In each of these disclosed methods, the wavelength of the transmitted laser beam must be chosen to detect a specific gaseous pollutant. Consequently, a second different pollutant could be detected on the same device and discriminated from the first pollutant only if a laser beam having a wavelength different from that used to detect the first pollutant were used. It would not therefore be possible to continuously monitor the atmosphere for several hazardous materials with a single device by the process disclosed in these two patents. In fact, continuous monitoring for more than one pollutant would be possible only by using as many devices as there are gaseous pollutants possible in the monitored area.
U.S. Pat. Nos. 4,490,043 and 4,529,317 each disclose a method for monitoring gaseous pollutants in which multiple transmitted laser beams are employed. In U.S. Pat. No. 4,490,043, two laser beams at different frequencies are employed. One beam has a wavelength specific to the gas or gases being monitored and the other beam is a reference beam at a nearby wavelength which does not have absorption bands from the pollutant or other compounds. This method is limited to detection of a specific pollutant or pollutants whose presence in the environment is expected. Further, this method requires variation of the amount of radiation reaching the detector in order to prevent overload to the detector when the beam scans areas of high reflectivity, thus limiting the quantitative capabilities of the device. Furthermore, this method gives only the direction of the gas and not the distance of a gas cloud.
In the monitoring method disclosed in U.S. Pat. No. 4,529,317, at least two scanning beams, each containing multi-plexed reference and measuring wavelengths are directed towards the location to be monitored from two spaced apart scanning positions. Combination of information from the two signals provides a measure of the amount of gas at the intersection of the two beams. As in the method disclosed in U.S. Pat. No. 4,490,043, the reference beam wavelength is selected on the basis of the particular pollutant expected to be present in the environment. Continuous monitoring for the presence of more than one pollutant can not therefore be accomplished with a single device.
In each of the above-described systems and any other infrared laser-based systems, only a limited number of discrete wavelengths can be generated by the laser and used for measuring and reference beams. The available wavelengths may not however be well matched to the absorption peaks of interest resulting in lower sensitivity of the system and increasing the likelihood of interference from other pollutants. In addition, since only one measuring wavelength is normally considered, any other compound which absorbs at the selected frequency would give a signal which would be indistinguishable from the signals given by the pollutant of interest. Interference in the measurement of the pollutant being monitored would result.
The Environmental Protection Agency has pursued methods for monitoring the atmosphere for gaseous pollutants which would not be limited to detection of only one compound at a time. A brief history of the Environmental Protection Agency's attempts to use remote optical sensing of emissions (ROSE) systems in which commercial Fourier Transform infrared spectrophotometers are used is given by Herget et al in "Remote Fourier Transform Infrared Air Pollution Studies," Optical Engineering, Volume 19, No. 4, pages 508-514 (July/August 1980). Herget et al also describes an improved mobile ROSE system in this article. Actual tests conducted with this improved mobile system are described in Herget's "Remote and Cross-stack Measurement of Stack Gas Concentrations Using a Mobile FTIR System" in Applied Optics, Vol. 21, No. 4, pages 635-641 (Feb. 15, 1982).
One of the EPA's first ROSE systems was a mobile system designed to detect and measure pollutants present at various pollution sources from a point remote from the pollution source. In this system, interferograms collected in the field were returned to a central computer for processing. This delay between data collection and data processing was undesirable because it made it difficult, if not impossible, to detect a leak before it had spread and created a potentially hazardous condition. This early system was subsequently improved to permit on-site processing. However, both the original and improved systems employed a grating monochromator which did not have the spectral resolution or optical efficiency necessary to detect trace amounts of pollutants.
Herget et al reports that these ROSE systems were subsequently further modified to include an interferometer system capable of detecting wavelengths in the infrared spectral region of from 650 to 6000 cm.sup.-1. This ROSE system could be transported in a van-sized vehicle to a location near the area to be monitored. In this most recent ROSE system, a light source and a source telescope are first positioned at a site remote from the site to be monitored. Energy from this remote light source is collected by a receiver telescope through an opening in the van wall. A tracking mirror reflects the infrared signal of gas(es) emitted from a stack of the facility being monitored into the receiving telescope. The receiving telescope focuses energy from the remote light source and the infrared signal of the gas(es) at the interferometer aperture. The interferometer detector which has a dual element sandwich-type configuration mounted in a liquid nitrogen Dewar scans two infrared regions (i.e. 1800 to 6000 cm.sup.-1 and 600 to 1800 cm.sup.-1) separately. Selection of the desired detector element is made by command (via computer). Two beamsplitters are employed in the interferometer. A beamsplitter interchange and realignment takes about five minutes.
This most recent ROSE system has several practical disadvantages. For example, this system employs a stationary source light and source telescope which are positioned in a manner such that the source light is reflected off of the tracking mirror to the receiver telescope. Movement of the tracking mirror to monitor a second nearby site thus necessitates movement of the source light and source telescope if an accurate measurement is to be obtained. This ROSE system is therefore incapable of monitoring more than one site by simply adjusting the tracking mirror. Nor can such system be modified in a simple manner to permit monitoring of several different locations within a few minutes. In fact, Herget et al states that after the initial setup of the ROSE system (which takes two to three hours), subsequent setups in the same vicinity require about one hour under normal conditions.
This newest ROSE system is also incapable of giving precise readings on pollutants which are not in line with or which have already crossed the optical path defined by the remote light source.
Further, pollutants which do not exhibit characteristic infrared peaks in the range of the selected beamsplitter (i.e. either 650-1800 cm.sup.-1 or 1800 to 6000 cm.sup.-1) can not be identified without a beamsplitter interchange and realignment. Such interchange and realignment, however, take about five minutes. Consequently, if two pollutants were present in a monitored area simultaneously and one of those pollutants had a characteristic IR band at 1700 cm.sup.-1 and the other had a characteristic band at 1900 cm.sup.-1, the ROSE system could not detect and monitor both pollutants simultaneously.
Yet another disadvantage of the ROSE system is the initial set up tie of two to three hours. The length of time required for set up makes the ROSE system almost useless for detecting a leak in its early stages unless the system is permanently installed at the location of the leak or a leak happens to occur during the scheduled monitoring of an area.
Finally, the primary mirrors of the ROSE System are not suitable for continuous exposure to corrosive industrial atmospheres.
Another improved system for monitoring gaseous pollutants is presently being marketed by Bomem Corporation. Detailed information with respect to the construction and operation of this Bomem system are not yet available to Applicants. However, it is apparent from advertising literature for this system that the optical components used in this system and the design of the instrument would not withstand the corrosive atmospheres encountered in many industrial environments.